1. Field of the Invention
The present invention relates to a communication system, in particular, relates to a communication system using quasi-synchronous detection for use in a high speed burst communication over a modulation transmission line.
2. Description of the Related Art
Since time division multiplex access (TDMA) using a modulation transmission line can more flexibly change information transmission speeds of child stations with a single modem than frequency-division multiple access (FDMA), the TDMA has been widely studied, developed, and utilized for fixed and mobile radio communications, and communications over tree shaped networks using coaxial cables.
With respect to the modulation and demodulation, one of technical problems to be solved to enhance the efficiency of the TDMA is how to remove the acquisition of carrier synchronism. In other words, by removing a carrier synchronous field of each burst signal transmitted from each station, the efficiency of using frames can be enhanced.
A technique for solving such a problem which has been used is asynchronous detection typified by delay detection. However, in the delay detection, a delay amount with a satisfactorily high accuracy should be provided for 1/fc sec where fc is the frequency of an IF carrier. Moreover, the delay detection is inferior to the synchronous detection with respect to the C/N to bit error rate (BER) property. It is known that when for example a four-phased phase modulation signal is detected, the delay detection is inferior to the synchronous detection theoretically by 2.3 dB in the C/N ratio for obtaining the same BER. As the modulation system uses many values and many phases, the deterioration of the C/N ratio in the delay detection tends to correspondingly increase.
Recently, a system having the same C/N to BER property as the synchronous detection without necessity of an acquisition field of carrier synchronism for each burst has attracted public attention and studied. In this system, many points of a quasi-synchronous detection signal are sampled and then stored in memory. Thereafter, the stored signal is digitally processed and then a clock, carrier frequency, and carrier phase rotation amount are estimated so as to demodulate the data.
The quasi-synchronous detection is a method for freely running an oscillated signal of a local oscillator used for converting an IF signal into a base band signal in the vicinity of the IF carrier frequency without feeding back the signal like the Costas system or the PLL system.
In this system, of course, it is not assured that the in-phase component (Is) and the quadrature component (Qs) are perfectly separated. Generally, a vector (Is, Qs) is obtained by rotating the phase of a vector (Ic, Qc) by an angle .theta.. where Ic is an in-phase component and Qc is an quadrature component. In addition, 8 can be represented with for example the following linear function for a time t. EQU .theta.=2.pi..times.t.times.(fL-fC)+.theta..sub.O
where .theta..sub.O is a constant specific to each burst, fc is a carrier frequency of an input IF signal, fL is a local oscillation frequency.
In the quasi-synchronous detection system which has been used, since the bit synchronization for the burst signal is not established, before a burst signal is received, the burst signal should be sampled at many points by using a clock with a frequency which is at least several times as high as the transmission speed (baud rate) of the signal as described above.
The sampling at many points results in increasing the data amount and requiring to estimate a timing, phase, and, when necessary, frequency from a large amount of data. Thus, since the calculation amount becomes huge, the signal speed is significantly affected by the processing capacity of the calculation system.
Practically, in a demodulator according to the quasi-synchronous detection method which was reported in Spring Conference, The Institute of Electronics, Information and Communication of Japan, held on March 1989, a digital signal processing LSI could demodulate a four-phased phase modulation signal of at most 250 k baud.
As described above, when the quasi-synchronous detection having the same bit error rate as the synchronous detection system without necessity of a field for acquiring carrier synchronism is performed, since an optimum sampling timing cannot be known before a burst signal is received, the burst signal should be sampled at many points and the huge data being obtained should be processed in complicated ways. Thus, the device becomes complicated and high speed burst communication becomes difficult.
The present invention was made to solve the above mentioned problems.